Semiconductive translator



H. E. HARING 'sEMxcoNDucTIvE TRANsLAToR filed Dec. so, 195s ,'Dec. 17, 1957 O/Y/DANT COA TING /Nl/ENTOR H. E. HAR/N0 IWW/M C ATTORNEY United States Patent y SEMICONDUCTIVE T RANSLATOR Horace E. Haring, Summit, N. J., assgnor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application December 30, 1953, Serial No. 401,419

2 Claims. (Cl. 148-33) This invention involves the use on semiconductive devices of particular coatings containing ions of appropriate charge so as to render and maintain the semiconductive Surface in the desired condition as to conductivity type.

There is recognition in the art of the degradation of the performance of semiconductor devices resulting from the existence of leakage paths on or near the surface of the semiconductive body. Generally, several schools of thought exist to explain the phenomena of these leakage paths. Material of a general nature relating to surface phenomena of semiconductors, specifically germanium, by W. H. Brattain and J. Bardeen is found in the Bell System Technical Journal, vol. 32, No. 1, I anuary 1953, pages 1-41.

One of the several theories regarding surface phenomena is disclosed in a paper by W. L. Brown appearing in the Physical Review, vol. 91, No. 3, August 1, 1953, page 518. As set forth herein, the occurrence of an extra conducting path, termed a channel, in a crystal of semiconductive material appears to be a consequence of the existence of a charge on the surface of the material. A positive charge on the surface of a P-type crystal induces a net negative charge within the crystal adjacent to the surface. This negative space charge or net deficit of positive charges is provided in part by the loss of holes from the valence band near the surface of the crystal and in part by the presence of electrons in the conduction band. It is these electrons which form a layer of N-type conductivity at the semiconductor surface. Conversely the existence of a negative surface charge on N-type material may result in a layer of P-type conductivity. These conductivity layers then function as extra conducting paths or channels.

According to another widely held view regarding the mechanism controlled by the practice of this invention, the unwanted leakage paths are formed either by the electrons associated with adsorbed positive ions or by the positive holes associated With adsorbed negative ions. In this instance the term bridges, has been applied to describe the nature of the conductivity. Thus, for example, in the presence of moisture, hydrogen ions are electrolytically discharged on the surface of germanium as a result of the reaction between germanium and water. It appears that the discharged hydrogen is present as positively charged hydrogen ions (H+) and their associated conductivity electrons. Considering the effeet of this increase in the surface concentration of positive ionsL and conductivity electrons in relation to the type of germanium involved indicates the nature of objectionable leakage paths. At the surface of P-type germanium, which is P-type because it contains a slight fice excess of negative ions, such an increase generally more' than neutralizes the slight excess of negative ions which make it P-type thereby establishing an N-type layer on P-type germanium. Such a layer will constitute an extra conducting path or bridge.

Finally, there is some thought which holds that water vapor or moisture itself provides the unwanted surface leakage path. According to this view water vapor adsorbed at the surface provides a leakage path as a result of the conductivity of the moisture itself.

While there is divergence of opinion regarding the underlying theory therefor, the existence of such extraconducting paths and indications that the environment contributes to their creation is recognized. However, there has been little success, so far as applicant is aware, in controlling or eliminating their deleterious eiects. The expedient or hermetic sealing not only is not" satisfactory but carries with it disadvantages both froin the standpoint of manufacturing techniques, and a less rugged and reliable device structure. In semiconductor devices of the junction-type such as disclosed in Patents 2,569,- 347, granted September 25, 1951, to W. Shockley, and 2,567,970, granted September 18, 1951, to J. H. Scaif et al., is is apparent that an extra-conducting path will act as a shunt or unwanted leakage path. Thus, in transistors of the junction-type leakage pathsacross the base zone, such as may be caused by moisture, will render the device extremely unstable and in single junction devices Will result in'undesirably high reverse currents.

This invention involves a surface treatment of the semiconductive material which comprises the application of a surface coating containing an agent capable of transferring ions of the appropriate charge to the semiconductor surface thereby to control the surface potential and the surface conductivity type, and thus avoid the formation of eXtra-conducting paths regardless of the moisture content of the ambient atmosphere.

It is an object of this invention to produce semiconductor devices having improved performance characteristics and stability. Specifically, an object is to inhibit the formation of deleterious leakage paths in semiconductor devices of the junction-type.

It is a further object of this invention to facilitate the manufacture of semiconductor devices. More specical- 1y, it is an object to simplify the production of semiconductor devices by eliminating the need for hermeticsealing or encapsulation.

According to this invention, improved semiconductor devices are attained by applying to the clean external surfaces of the assembled devices a coating of an inhibiting primer. For example, a suitable material for coating junction transistors of the N-P-N typemay comprise a mixture of red lead (Pb304) and a vehicle composed of polyethylene and polybutene. Similarly,`zine chromate or strontium chromate may be substituted for read lead with equally satisfactory results. Other oxidants may also be used. In certain applications it may be found advantageous to apply the coating only to the intermediate or base zone.

A remarkable feature of the structure 'attained by the practice of this invention is the permanence of the surface stabilization. Devices provided with the inhibitive coatings of this invention remain permanently free from bridges regardless of the moisture content of the ambient atmosphere.

A better understanding of the marked success attendant upon this invention may be obtained by a consideration of the best present theory supported by practical investigation regarding the phenomena associated therewith,

Without necessarily subscribingto. any. one of. theaforementioned theories regarding surface mechanisms, it can be said that the existence of channels or leakage paths is attributable to afreversal. of conductivity typeat the surface, or edge of a layer of one'conductivity type, for examplean N-type surface layer, onthe edge ofthe P-type layer of an N-P-N germanium junctiontransistor. Experimentation and theory indicates that this N-type layer is established on theexposed-surface of the P-type layer bythe adsorption of hydrogen ions (H+) made available by the reaction between the germanium and moisture upon its surface. Instead of therelatively unsuccessful and costly techniques heretofore followed of attempting to completely exclude'moisture, this. invention involves the continuous inhibition of .the hydrogen-forming reaction andthe complete: removal offhydrogen adsorbed during the cleaning process, by providingaV surface coating of a material which will furnish oxygenfor adsorptionon the germaniumi surface. The surface is thereby converted from an unstable N-type to a'stable P-type as a resultof the replacement of hydrogen ions (H+). by oxygen ions (O- Inv this way, the: effectiveenergy level of the surface is fixed so that the surface conductivity type is thesameas that of the adjacentfbulkvmaterial regardless of the moisturecontent of the ambient atmosphere. Under such circumstances the formation of the objectionable bridge or leakage path is impossible.

Having thus recognized the essentiality of the. oxygenated surface conditionas (l) anfantidote for moisture and the effects' of moisture and A(2) a stabilizing inuence inthe absence of .moisture, applicant has by his invention proceeded to the attainment of devices wherein this advantageous structure is realized.

A better understanding of this advantageous structure will be4 hadfrom .the following dissertation taken in connectionwith thedrawing, in which:

Fig. l showsin section one embodiment of this invention;v

Fig. 2 is another section of the embodiment of Fig. 1; and.

Fig. 3 shows in section` another embodiment of this invention.

Referring to Fig.` 1, there is shown in longitudinal section a lilamentary semiconductive!deviceofV the junction type such as is disclosed'in the aforementioned patent to Shockley. The device comprises .a layer or.zone11" of P-type material, such as germanium or silicon, interposed between two layers or zones 12`an`d`13fof` 'N-type material ofthesame composition as that 'of .theP-type zone, separated respectively by barriers 14 and'lS.' Connections are made to each layer by electrodesterm'ed emitter (E), collector (C), and base. (B). The Aentire'b'ody is encased in' al coating` 16' composed and appliedas hereinafter set forth: Fig. Z'isasection taken along the kline 2-2 of Fig.: l throughthe'base region to showA the adherence offthe'oxidant coating '16 to' the entire periphery ofthelarnent.

Fig. 3 is a longitudinal section of a-Semiconductive device identical to thatV of Fig.' l -towhichthe'coating of this invention is appliedfonly overthebase'region'and a portion of the adjacent` N-'type regions -12 and'1-13'and includingfthe barriers 14-fand 15.1 Such'a technique requires less coatingmaterial .and.1leaves the-lam'entends:freeioff coating for possible fabrication steps..

In preparing semiconductive.translatingdevices-:in accordance with thisinventioncertainattributes vof the inhibiting coating .shouldvobtainv The inhibiting agent',s such as for exampleredlead,` shouldbe non-conductive.: Where a vehicleis used. the agentshould not reactwithvfthevvehicle so as to deteriorate the iilr'n. In the presence of moisture, the water solubility ofthe agent should be sufficient top rovidc a uniform and continuous'oxidizingenviron- 4ment at. theinterface between thesemicondctorland" the coating. The solubility inust'not be so great however, as 'to lead'to leaching and 'consequent electrical leakage; This is not to be interpreted to indicate that moisture is essential to a proper conditioning of the surface. Stable surfaces of the desired potential and conductivity type can be established and maintained by inhibiting agents such as red lead regardless of the moisture content of the ambient atmosphere. It is desirable even where the semiconductive material is conned within an atmosphere of a high degree of dryness to provide an environment of a definite character in order to firmly tix the surface in the desired condition and thus stabilize device characteristics.

ln the coated devices of this invention, the abovementioned oxidizing agents, namely red lead (Pb304), zinc chromate (ZnCrO4), and strontium chromate (SrCrO4) are generally employed in the form of powdered pigments. In order to place the inhibitor in contact with the semiconductive surface in as homogeneous a fashion as possible it is usually desirable to provide a vehicle which will enable and maintain such contact. For example, gel-type, hot-melt coatings of the class composed of lowl molecular weight polyethylene and poly'butene mixtures may be used with the pigment type inhibitors heretofore mentioned. Mixture is readily accomplished by adding the polyethylene to the polybutene at a temperature of the order of 160 C. with thorough stirring. Likewise, the epoxide resins and other well-known paint vehicles may be employed.

ln selecting a satisfactory vehicle certain properties should be considered. It should adhere well to the surface of the-semiconductive material, hold the inhibiting agent in a finely'dispersed condition, have a high electrical resistanceand. be reasonably impermeable to moisture. In addition, as was mentioned in connection with the inhibitors, reactionbetween the agent and vehicle is undesirable.

Generally, satisfactory oxidizing coatings containing pigmeut ltype oxidants, such as red lead may comprise about 25. percent by volume with the vehicle making up the balance of the finished coating. The percentage range of the oxidant may vary to the extent of fifteen or twenty above and below the above-determined figure.

The preparation of suitable coating material involves conventional practices for the grinding or powdering of pigment type oxidants which may then be mixed with a vehicle of the polyethylene-polybutene type by stirring at a temperature somewhat above the melting point, or by mixing on a paint mill at a temperature either above or below the melting point. The coating may then be applied to the device in any of a number of conventional ways such as dipping, brushing, or spraying at a temperature above the melting point. Generally, where vehicles of the polyethylene-polybutene type are used, elevated temperatures of the order of to 160 C. aredesirableto insure the proper fluidity for convenient mixing,l application, surface wetting, adhesion and coverage. If the-selectedvcoating material is one of the many varieties lusually referred to as paints, the oxidant may be dispersed. throughout the vehicle with the aid of a suitable solvent such as toluene and then applied at room temperature to the surface to be coated. Any of the conventional. methods of application may be employed; Amonggthe'paints which appear to be suitable for the purposeof this inventionis one with a polymeric vehicle known asepoxide resin. This vehicle, readily obtainable.commercially, is a product of the condensationv reaction between bisphenol A and epichlorhydrin. It may be prepared for mixing with the oxidizing pigment by thinning with toluene. Immediately prior to application a curingagent such as triethylamine or diethylene triamiiie may be added to cross link the polymer `and thushardenthe film. Baking at 50 to 60 C. for

several hours is recommended.

It" will be understood that the'semiconductive material should present a proper surface before coating. To this end the'material may be subjected to any of the chemicalor electrolytic etchingtechniques known to the art followed by rinsing in high purity water and thorough drying.

ln one specific example, an N-P-N junctiontype transistor of germanium as disclosed in the aforementioned patent to Shockley was coated as described below. The coating mixture comprised one part by volume of commercial grade 97 percent red lead to three parts by volume of a mixture consisting of 7.5 percent by Weight of low molecular weight polyethylene and 92.5 percent by weight of polybutene. Adequate dispersion of the oxidant was accomplished by thorough mixing at a temperature in the range of 125 to 160 C.

The N-P-N junction device was then dipped in the coating mixture at a temperature of 112 C. Several seconds of immersion was sufcient to bring the device to the temperature of the coating compound and assure thorough wetting. It was then withdrawn, allowed to cool and subjected to initial measurements.

Likewise, a similar procedure was followed, substituting however, zinc chromate (ZnCrO4) for red lead as the oxidant with completely satisfactory results.

ln another specific example, the procedure of the rst example was followed using, however, a vehicle composed entirely of polybutene.

In still another specific example, P-type germanium coated with an epoxide resin paint containing red lead was found to maintain a P-type surface regardless of the moisture content of the ambient atmosphere, thus demonstrating that this kind of coating also will stabilize the surface and protect semiconductor devices against moisture and its effects. A repetition of this test, using zinc chromate as the oxidizing agent, yielded the same result.

In further tests it was found that the surface of P-type silicon could be maintained P-type in the presence of moisture by means of the oxidizing coatings of this invention.

It may be found desirable, although not essential, to treat the semiconductor device in an aqueous solution of chromic acid or of a dichromate immediately prior to the coating step. It appears that this pretreatment accelerates establishment of the desired surface condition in those cases where surface cleaning prior to coating has not been of the best quality.

It may be remarked that other combinations may be employed to provide the surface environment in accordance with this invention. Thus, where the semiconductor device is to be enclosed in an envelope such as a metal can, an oxidant such as red lead or a dichromate may be employed suspended or dissolved in dielectric oil.

What is claimed is:

l. A semiconductive body selected from the group consisting of germanium and silicon having two zones of N-type conductivity and an intermediate zone of P-type conductivity, and a surface coating on said zone of P-type conductivity comprising an oxidizing agent selected from the group consisting of red lead, zinc chromate, and strontium chromate, dispersed in a vehicle consisting of a mixture of 8 percent polyethylene and 92 percent polyisobutylene for producing a substantially continuous oxidizing environment at the interface between said layer and said body.

2. A semiconductive body selected from the group consisting of germanium and silicon having two zones of Ntype conductivity and an intermediate zone of P-type conductivity, and a surface coating on said zone of P-type conductivity comprising red lead dispersed in a mixture of 8 percent polyethylene and 92 percent polyisobutylene for producing a substantially continuous oxidizing environment at the interface between said layer and said body.

References Cited in the le of this patent UNITED STATES PATENTS 2,349,622 Hewlett May 23, 1944 2,391,706 Jackson et al Dec. 25, 1945 2,438,110 Brattain Mar. 23, 1948 2,444,255 Hewlett June 29, 1948 2,569,892 Jones et al. Oct. 2, 1951 2,615,587 Clarke Oct. 28, 1952 2,655,626 Cepon Oct. 13, 1953 2,722,490 Haynes et al Nov. 8, 1955 2,743,430 Schultz Apr. 24, 1956 2,748,325 Jonny May 29, 1956 

1. A SEMICONDUCTIVE BODY SELECTED FROM THE GROUP CONSISTING OF GERMANIUM AND SILICON HAVING TWO ZONES OF N-TYPE CONDUCTIVITY AND AN INTERMEDIATE ZONE OF P-TYPE CONDUCTIVITY, AND A SURFACE COATING ON SAID ZONE OF P-TYPE CONDUCTIVITY COMPRISING AN OXIDIZING AGENT SELECTED FROM THE GROUP CONSISTING OF RED LEAD, ZINC CHROMATE, AND STRONTIUM CHROMATE, DISPERSED IN A VECHICLE CONSISTING OF A MIXTURE OF 8 PERCENT POLYETHYLENE AND 92 PERCENT POLYISOBUTYLENE FOR PRODUCING A SUBSTANTIALLY CONTINUOUS OXIDIZING ENVIRONMENT A T THE INTERFACE BETWEEN SAID LAYER AND SAID BODY. 